Four cruise-missile-equipped B-52Hs from Minot Air Force Base, N.D., will rotate with four of the eight B-52Hs already in theater, he added. Pentagon officials are also considering sending Army AH-64 Apache attack helicopters to join the fray, he said.

The B1-B Lancers, from Ellsworth Air Force Base, S.D., are slated to deploy and be ready for air operations by April 1, according to Pentagon officials. The all-weather, supersonic, intercontinental-range bombers can carry up to 84 Mark-82 conventional 500-pound bombs and 30 anti- armor/artillery cluster munitions.

"The air defense threat is changing all the time," he said. "Therefore, we have to change our methods all the time, and we're doing that. This is not a static situation. It is clear that suppression has been a very important part of this mission, and it is one that we want to continue and, in some places, augment."

The United States has lost one aircraft since Allied Force began March 24. An F-117A Nighthawk stealth fighter-bomber went down March 27 in northern Yugoslavia. A U.S. search team rescued the pilot several hours later.

The cause of the crash remains unknown, Bacon said. "It's not just a simple issue of determining what happened," he said. If investigators find that the Nighthawk was shot down by a missile, as the Serbs claim, the next questions are how and what that means for future NATO air operations.

"Because it's very operationally sensitive, we want to be very careful in pulling together as much information as we can about how it happened," he said. For this reason, he added, the information may be withheld from the public.

Nighthawks remain an integral part of Operation Allied Force, Bacon pointed out. "This reinforces the fact that this is an extraordinary aircraft. It has performed brilliantly over Iraq, and it's performing brilliantly today over Yugoslavia."

U.S. and NATO officials are making every reasonable effort to protect all allied planes and pilots, Bacon added. "We obviously have concerns about vulnerability, and that term transcends just technology. It applies to flight tactics. It applies to routes. It applies to a whole variety of things, and these are all among the issues that have to be determined in deciding, figuring out exactly why a plane goes down."

Sophisticated Yugoslavian air defenses are only one of three challenges, Bacon noted. Mountainous terrain and inclement weather also confront NATO Allied Force pilots. Bad weather is affecting missions, he said, but U.S. all- weather air- and sea-launched cruise missiles and precision-guided munitions can surmount some of the weather problems.